1. Field of the Invention
The present invention relates to a method for producing a semiconductor laser device used as a light source for performing reading operations from an optical disk. More particularly, the present invention relates to a method for producing a semiconductor laser device using a vapor phase epitaxy (VPE) method, where the thickness of a cap layer is increased to reduce tracking errors due to return light.
2. Description of the Related Art
In recent years, optical disks such as CDs (Compact Disks) and MDs (Mini Disks) have rapidly become popular, as they can provide high sound quality without noise while they do not deteriorate through wear, in contrast to conventional analog records.
In an analog record, a pick up (the tip of a stylus) traces a groove on the record. In an optical disk, the pick up is not in contact with the disk, whereby it is necessary to detect positions of the signal pits and to move the pick up along the disk to detect the signal pits.
At present, the three beam tracking servo mode (hereinafter, "three beam mode") is widely used for this purpose, where light from a light source is divided into a main beam and primary diffraction light.
However, the three beam mode has the following problems.
As shown in FIG. 5, laser light 102 is emitted from a laser chip 100, passes through a diffraction grating 101, is reflected by a half mirror 103, passes through an objective lens 104, and is incident upon an optical disk 105. Reference numeral 106 denotes a stem upon which the laser chip 100 resides. Then, the reflected light returns through the same optical path to be received by a light receiving section (not shown) after passing through the half mirror 103. However, a portion of the reflected light returns back to an emission end face of the laser chip 100 through the diffraction grating 101. This portion of light is reflected again (indicated at "A" and "A" in FIG. 5) by the end face of the laser chip 100 ("A" in FIG. 5), which adversely influences the signal detection.
The position on the chip end face upon which the reflected primary diffraction light will be incident varies depending upon the optical system used, but is typically about 60 .mu.m above or below the emission region.
Conventionally, to address this problem, the light emitting region (light emitting layer) of the laser chip 100 is located so that the primary diffraction light will not be incident upon the emission end face, as shown in FIG. 6A. Particularly, the light emitting region is located approximately in the middle of the semiconductor laser chip 100.
As shown in FIG. 6B, the specific thicknesses may be Le.ltoreq.60 .mu.m, Ls.ltoreq.60 .mu.m, where Le denotes the thickness of an epitaxial layer 101a above the light emitting region while Ls denotes the thickness of an epitaxial layer and the substrate below the light emitting region.
Herein, if the wafer thickness of the laser chip 100 is less than about 70 .mu.m, a crack or a chip in the wafer may easily occur, which hinders the fabrication process. Therefore, there is provided an additional condition: Le+Ls.gtoreq.70 .mu.m.
Thus, the thickness Le of the epitaxial layer above the light emitting region will be 10 .mu.m.ltoreq.Le .ltoreq.60 .mu.m.
The liquid phase epitaxy (LPE) method has been used to realize such a layer thickness in the conventional structure shown in FIGS. 6A and 6B. The LPE method has been used since it has a relatively high growth rate and is able to grow a thick film within a short period of time, thereby providing a better production efficiency, compared to the VPE method, which has a relatively low growth rate, thus requiring a longer period of time to grow a thick film.
However, in the LPE method, dopant impurities in the vicinity of the light emitting region diffuse due to the high growth temperature (up to about 800.degree. C.). This diffusion factor deteriorates the performance of the product.
Moreover, a quantum well structure obtained by depositing thin films having thicknesses of several to around twenty .mu.m, when exposed to such a high temperature, results in the migration of material atoms as well as the impurity diffusion, whereby the quantum well structure is destroyed. Thus, such a high temperature during the film growth phase hinders the production of a high performance semiconductor laser device using the quantum well structure.